High-strength concrete-like fluorogypsum-based blends and production method

ABSTRACT

High-strength concrete-like FG blends and methods for producing them are described. The blend includes FG, hydraulic cement, additional alkali material, and pozzolanic material. The blend further includes an admixture used in the formulation of concrete. The blend further includes an aggregate. The aggregate is a coarse aggregate or a fine aggregate.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. application Ser. No.16/129,515, filed Sep. 12, 2018, now U.S. Pat. No. 10,865,147, whichclaims the benefit of U.S. Provisional Patent Application No. 62/557,735filed Sep. 12, 2017, each of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to high-strength concrete-likefluorogypsum blends and methods for producing them.

BACKGROUND OF THE INVENTION

Fluorogypsum (FG) is an acidic (pH≈2.3) by-product of the industrialproduction of hydrofluoric acid from fluorspar. FG is discharged inslurry form from the producer and placed in settlement ponds until theFG hardens (King 1986; Azar 1990; Chesner et al. 1998). The hardened FGhas a low pH and needs to be neutralized in order to avoid potentiallyharmful properties such as corrosiveness (Chesner et al. 1998). Thisneutralization can be performed by adding a small amount (e.g., lessthan about 6% of dry weight) of alkaline material such as lime (e.g.,Eisele 2003) or circulating fluidized bed combustion ash (Lind 1999),and obtaining a new material referred to as blended calcium sulfate(Zhang and Tao 2006) or pH-adjusted FG. This material is then stockpiledin mounds, where it is exposed to weather and potential contaminantsbefore it is removed for potential use. The addition of alkalinematerial and the stockpiling of this material are not subjected toquality control. This material is referred to as uncontrolledpH-adjusted FG hereinafter.

Uncontrolled pH-adjusted FG has been considered for use in constructionapplications (Brink 1973; Usmen and Moulton 1984; Zhang and Tao 2006;Singh and Garg 2009; Wu et al. 2010). Uncontrolled pH-adjusted FG isreadily available, because it is stockpiled by the chemical plantsproducing hydrofluoric acid; and pure FG cannot be used as is because itpresents high levels of acidity and long setting times, which areconsidered undesirable properties for a construction material (Singh andGarg 2009).

Currently, FG is used only in the form of rocks of different grading(after neutralization and stockpiling) for subgrade of roads and parkinglots.

SUMMARY

In an embodiment of the invention, a pre-cure composition includes FGand hydraulic cement. The FG can be in slurry form, such as a wet slurryform or a dry slurry form. The pre-cure composition can include anadditional alkali material. For example, the additional alkali materialcan be lime, a coal combustion product, circulating fluidized bedcombustion ash (CFBCA), or a combination.

The pre-cure composition can also include a pozzolanic material. Forexample, the pozzolanic material can be a solid fuel combustion product,a coal combustion product, fly ash, class C fly ash, class F fly ash,bottom ash, flue-gas desulfurization materials, boiler slag, incineratorbottom ash, a biomass combustion product, bagasse ash, rice hull ash,wood ash, biomass pellets ash, natural pozzolan, volcanic ash, anindustrial amorphous silica product, micro-silica, silica fumes, or acombination. In some embodiments, the pozzolanic material is a solidfuel combustion product, a coal combustion product, fly ash, class C flyash, class F fly ash, bottom ash, flue-gas desulfurization materials,boiler slag, incinerator bottom ash, or a combination thereof. In otherembodiments, the pozzolanic material is a biomass combustion product,bagasse ash, rice hull ash, wood ash, biomass pellets ash, or acombination thereof. In still other embodiments, the pozzolanic materialis natural pozzolan, volcanic ash, or a combination thereof. In otherembodiments, the pozzolanic material is an industrial amorphous silicaproduct, micro-silica, silica fumes, or a combination thereof.

The pre-cure composition can also include an admixture used in theformulation of concrete. For example, the admixture can be a materialwith latent hydraulic behavior, ground granulated blast furnace slag, awater-reducing agent, entrained air (or another gas), asuperplasticizer, a set retarding agent, a set accelerating agent, ashrinkage-reducing agent, or a combination.

The pre-cure composition can also include an aggregate. For example, theaggregate can be a coarse aggregate (such as gravel, crushed stone, orrubble) or a fine aggregate (such as sand).

The solids in the FG can include at least about 10 wt % anhydrite, atleast about 25 wt % anhydrite, at least about 50 wt % anhydrite, or atleast about 70 wt % anhydrite (CaSO₄). The FG can have a pH in the rangeof from about 1.5 to about 5.5, from about 1.5 to about 5, from about1.5 to about 3.5, from about 2 to about 3, from about 2 to about 2.8,from about 2 to about 2.6, from about 2.1 to about 2.5, from about 2.2to about 2.4, from about 2.25 to about 2.35, or of about 2.3.

The pre-cure composition can include hydraulic cement and/o othercementitious materials. For example, the hydraulic cement can bePortland cement, Type I Portland cement, Type II Portland cement, TypeIII Portland cement, Type IV Portland cement, Type V Portland cement, ora combination.

For example, the pre-cure composition can include at least about 45 wt %FG, at most about 10 wt % hydraulic cement (e.g., from about 0.01 wt %to about 10 wt %), at most about 12 wt % additional alkali material(e.g., from about 0.01 wt % to about 12 wt %), and at most about 40 wt %pozzolanic material (e.g., from about 0.01 wt % to about 40 wt %). Forexample, the pre-cure composition can include from about 58 to about 62wt % FG, from about 0 (e.g., 0.01 wt %) to about 4 wt % circulatingfluidized bed combustion ash, from about 34 to about 35 wt % fly ash,and from about 3 to about 6 wt % Portland cement. For example, thepre-cure composition can include at least about 5, 10, 15, 20, 25, 30,35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 98 wt % FG. Forexample, the pre-cure composition can include at most about 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65,70, 75, 80, 85, or 90 wt % hydraulic cement (e.g., from about 0.01 wt %to about 90 wt %). For example, the pre-cure composition can include atmost about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 25, 30, 35,40, 45, or 50 wt % additional alkali material (e.g., from about 0.01 wt% to about 50 wt %). For example, the pre-cure composition can includeat most about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 25, 30, 35,40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95 wt % pozzolanicmaterial (e.g., from about 0.01 wt % to about 95 wt %).

In a method according to the invention, the pre-cure composition is usedas a partial or total substitute of cement to produce concrete. Forexample, the produced concrete can include fine and/or coarse aggregate.

In a process according to the invention, a FG-based concrete-likematerial is formed by mixing a pre-cure composition according to anyembodiment disclosed herein to form a dry mixture, mixing the drymixture with water to form a wet blend, and allowing the wet blend tocure to yield the FG-based concrete-like material. For example, the FGcan be first mixed with the additional alkali material to yield treatedFG; and the treated FG can be second mixed with the hydraulic cement toyield the dry mixture. For example, the FG can be first mixed with CFBCAto yield treated FG; the treated FG can be second mixed with fly ash andPortland cement to yield the dry mixture; and the dry mixture can bethird mixed with water at a water over dry material ratio of less thanabout ⅓ to form the wet blend. For example, the FG can be first mixedwith the additional alkali material to yield treated FG; the treated FGcan be secondly mixed with the pozzolanic material and Portland cementto yield the dry mixture; the dry mixture can be thirdly mixed with theadmixture used in the formulation of concrete to yield a second drymixture; and the second dry mixture can be fourthly mixed with water toyield the wet blend. For example, the FG can be first mixed with theadditional alkali material to yield treated FG; the treated FG can besecondly mixed with the pozzolanic material and Portland cement to formthe dry mixture; the dry mixture can be thirdly mixed with the aggregate(such as a fine aggregate or a coarse aggregate) to yield an aggregatedmixture; and the aggregated mixture can be fourth mixed with water toform the wet blend. For example, the treated FG can have a pH of lessthan about 2.4, less than about 2.5, less than about 3, less than about3.5, less than about 4, less than about 5, less than about 6, or lessthan about 6.5. For example, the treated FG can have a pH of from about6.5 to about 7. For example, the treated FG can have a pH of from about6 to about 7.5. For example, the wet blend can be allowed to cure for atleast about 28 days to form the FG-based concrete-like material. Forexample, the FG-based concrete-like material can have a compressivestrength of at least at least about 20 MPa, at least about 30 MPa, atleast about 40 MPa, at least about 50 MPa, or at least about 60 MPa. Forexample, the FG can be in a wet slurry form. For example, the FG can bein a dried slurry form.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates X-ray diffractogram of an FG sample (G: gypsum, A:anhydrate, F: fluorite).

FIG. 2 illustrates X-ray diffractogram of a CFBCA sample (G: gypsum, C:calcite, E: ettringite).

FIG. 3 illustrates effects of CFBCA content on compressive strength ofFG-based blends.

FIG. 4 illustrates effects of CFBCA content on unit weight of FG-basedblends.

DETAILED DESCRIPTION

Embodiments of the invention are discussed in detail below. Indescribing embodiments, specific terminology is employed for the sake ofclarity. However, the invention is not intended to be limited to thespecific terminology so selected. A person skilled in the relevant artwill recognize that other equivalent parts can be employed and othermethods developed without parting from the spirit and scope of theinvention. All references cited herein are incorporated by reference asif each had been individually incorporated.

Millions of tons of solid by-product materials are produced every yearby chemical industries all over the world. The accumulation of thesematerials causes substantial societal costs for containment anddisposal, including environmental pollution and related economic losses(UNEP 2005). Therefore, finding new beneficial applications for theselarge reserves of unused and/or underutilized materials is of greatinterest and provides important opportunities for sustainable economicdevelopment. At the same time, the construction industry is alwayssearching for alternative supplies of usable materials in order to curbits carbon footprint, reduce the cost of new projects, and ensurelong-term sustainability of the industry itself (Halstead 1979; Cliftonet al. 1980a, 1980b; Ciullo 1996; Worrell et al. 2001).

Among the different options that have been investigated in the last fewdecades, significant attention has been paid to the utilization ofgypsum-based by-product materials in the construction industry (Brink1973; Sajwan et al. 2006; Zhang and Tao 2006). One of these gypsumby-products is FG, which is an acidic by-product of the industrialproduction of hydrofluoric acid from fluorspar. FG is discharged inslurry form from the producer and placed in settlement ponds until theFG hardens (King 1986; Azar 1990; Chesner et al. 1998). The hardened FGhas a very low pH and needs to be neutralized in order to avoidpotentially harmful properties such as corrosiveness (Chesner et al.1998). This neutralization is usually performed by adding a small amount(e.g., about 2% to about 6% of dry weight) of alkaline materials such aspure lime (Eisele 2003) or CFBCA (Lind 1999), and obtaining a newmaterial referred to as blended calcium sulfate (Zhang and Tao 2006) orpH-adjusted FG (Bigdeli et al. 2018a). This material is then stockpiledin mounds, where it is exposed to weather and potential contaminantsbefore it is removed for potential use. The composition of the naturalfluorspar, the addition of alkaline materials, and the stockpiling ofthis material are not subjected to quality control. Thus, differentbatches of the resulting material can have very different chemical andphysical properties even when produced by the same chemical plant. Thismaterial is referred to as uncontrolled pH-adjusted FG (U-FG)hereinafter.

However, the usage of U-FG in both experimental research and real-worldconstruction applications is often associated with a wide variability ofthe experimental results and performance of the materials (Bigdeli etal. 2018a), mainly due to the following issues: (1) the composition ofthe natural base fluorspar varies between different batches; (2) U-FGcan present a high variability in the alkaline material content betweendifferent batches and even within the same batch due to thenon-uniformity of the lime/CFBCA treatment (both in time and space) andthe usage of alkaline materials with different levels of purity; and (3)the chemical, physical, and mechanical properties of U-FG can beinfluenced by the exposure to contaminants and environmental phenomena,such as temperature changes, precipitation, and freezing/thawing cycles,which depend both on the location and the duration of the stockpilingbefore use of this material.

FG-based blends can be introduced as binders in which high percentagesof Portland cement and low percentages of FG are used. Garg and Pundir(2014) used pure FG mixed with a small (e.g., about 0.5% to about 1.0%in weight) quantity of lime to investigate the feasibility of using ablend of pH-adjusted FG, granulated blast furnace slag, and Portlandcement (PC) as a composite binder for outdoor construction applications.Yan and You (1997) and Yan et al. (1999) also used pure FG mixed withlarge amounts (i.e., >about 50% of dry weight) of fly ash (FA) andactivated with PC to obtain a cementitious binder (Yan and You 1997).

The present invention provides a concrete-like material with acomposition of slurry FG (FG) that may be treated using alkali materials(e.g., lime and circulating fluidized bed combustion ash), pozzolanicmaterials (e.g., metakaolin, microsilica, silica fume, fly ash,rice-husk ask, bagasse ash) and hydraulic cement (e.g., Portlandcement), which is referred hereinafter as FG-based blend (Bigdeli et al.2018b). In some embodiments, the FG-based blend may have a weightpercentage of the FG that is greater than a weight percentage of thehydraulic cement. Additional additives commonly used with ordinaryconcrete (including, but not limited to materials with latent hydraulicbehavior such as ground granulated blast furnace slag,superplasticizers, water-reducing agents, air-entrainment,set-retarding/accelerating, shrinkage reducing admixtures) can also beadded to the blend. Unlike ordinary concrete, FG-based blend does notneed to contain aggregates; however, it can be used as a directsubstitute of ordinary concrete in construction applications. Thepresent invention offers an alternative production method and a set ofcompositions with high compressive strength, high FG content, and lowPortland cement content (Bigdeli et al. 2018b). The high compressivestrength is a highly desirable property for construction applications,whereas minimizing the amount of Portland cement in the compositionresults in lower production costs.

The invention describes the compositions and methods used to produce aconcrete-like material that can be used as a substitute of ordinaryconcrete. This invention uses slurry FG, which is currently consideredan industrial waste.

The present invention introduces a method to: (1) use slurry FG (FG) asproduced by chemical plants, (2) treat its pH with small amounts ofalkali materials (less than or equal to the amount needed for fullneutralization), (3) mix the treated FG (e.g., ≥about 50% in weight ofthe dry material) with hydraulic cement (e.g., about ≤10% in weight ofthe dry material) and pozzolanic materials (e.g., about ≤40% in weightof the dry material), and (4) mix with water at a water over drymaterial ratio lower than about 1/3. The obtained material, referred toas FG-based blend, has a high compressive strength (e.g., from about20-about 50 MPa-about 3000-about 7000 psi) and modulus of elasticity(e.g., from about 18-about 21 GPa-about 2600-about 3000 ksi). FG-basedblends can be used as direct substitute of ordinary unreinforcedconcrete in construction applications.

As disclosed herein, embodiments of the FG-based blend comprises atleast about 45 wt % FG, and at most about 10 wt % (e.g., from about 0.01wt % to about 10 wt %) hydraulic cement. The FG-based blend may furthercomprise at most about 2 wt % (e.g., from about 0.01 wt % to about 2 wt%) additional alkali material. In addition, the FG-based blend may alsocomprise at most 40 wt % (e.g., from about 0.01 wt % to 40 wt %)pozzolanic material.

In some embodiments, the FG-based blend comprises from about 60 wt % toabout 90 wt % FG, from about 0 wt % (for example about 0.01 wt %) toabout 38 wt % FA and from about 2 wt % to about 10 wt % hydrauliccement, such as Portland cement

The FG-based blend can develop compressive strength, modulus ofelasticity, and setting time that are similar to those of ordinaryconcrete, with a lower unit weight (e.g., from about 1800-about 2100kg/m³) and lower amounts of hydraulic cement, which corresponds to loweremission of CO₂ during the production of this material. The FG-basedblend that is the subject of this application enables higher usage ofby-product FG, which contributes to the sustainability of this materialand minimizes costs associated with FG neutralization and stockpiling,because it directly uses the slurry FG.

Embodiments of the invention represent a lower-cost alternative toordinary Portland cement concrete. In terms of cost per unit volume(cubic yard), embodiments can be produced at a cost of about $50-about65/cu yd., less than a typical cost of about $75/cu yd. of ordinaryconcrete.

The FG-based blend according to the invention can be used as a directsubstitute of unreinforced concrete in several applications, including(but not limited to): bearing walls, earth retaining walls,slab-on-grade construction, shallow foundation elements not requiringsteel reinforcement, etc.; concrete blocks, concrete panels,pre-fabricated elements, etc.; road pavement, pavement for parking lotsand walking areas, sidewalks, etc.; coastal protection structures suchas sea walls and water breakers; and as a bagged construction materialfor small jobs in dry powder form (similar to concrete mix bags).

The mechanical and durability properties of embodiments of theinvention, that is, blends made of FG (FG) with the pH adjusted by usingcontrolled amounts of CFBCA and denoted as C-FG, class C fly ash (FA),and type II Portland cement (PC), have been considered (Bigdeli et al.2018b). A series of pH tests was conducted on samples of C-FG to developan analytical relationship between acidity and CFBCA content, which canbe used to determine the optimal amount of CFBCA needed to obtain aspecified pH value. Two compositions of C-FG-based blends wereinvestigated in detail to identify the effects of CFBCA content oncompressive strength, modulus of elasticity, Poisson's ratio, relativevolumetric expansion, unit weight, and setting times (Bigdeli et al.2018b). The obtained properties were compared with those of FG-basedblends having the same composition and made using FG with the pHadjusted by using uncontrolled amounts of CFBCA (U-FG). Results suggestthat the amount of CFBCA can have significant effects on the propertiesof C-FG-based blends, depending on the composition. In addition,C-FG-based blends generally achieve a higher compressive strength andinitial stiffness than the corresponding U-FG-based blends (Bigdeli etal. 2018b).

The addition of controlled amounts of CFBCA to FG in slurry form, toobtain pH-adjusted FG, referred to as controlled pH-adjusted FG (C-FG)hereinafter, for the production of FG-blends containing PC and FA wasconsidered. CFBCA was considered, as well as lime, because CFBCA is acheaper alternative to lime. The FG-based blends made by utilizing thisC-FG material are referred to as C-FG-based blends, in order todistinguish them from FG-based blends made by using U-FG material, whichare referred to as U-FG-based blends hereinafter.

There are the following advantages of using C-FG-based blends versusU-FG-based blends: (1) the sources of variability for the mechanical andphysical properties of C-FG are reduced to the natural variability ofthe base fluorspar only, which allows for the production of constructionmaterials with reproducible properties; (2) the quality control forconstruction materials made using C-FG is easier to implement than thatfor materials made using U-FG; and (3) the production costs ofC-FG-based blends can be reduced when compared to that of U-FG-basedblends, because the costs associated with pH neutralization,stockpiling, and maintenance can be minimized or avoided. Thus,embodiments according to the invention represent the development of aneconomical and sustainable substitute of ordinary concrete using theindustrial by-product FG.

Experimental tests show that the slurry FG used may have a water contentof about 20% by weight and a pH of about 2.28. The slurry FG used inthis research is left to air dry and solidify for four days and then isground and sieved by using a US standard sieve #10 (corresponding to amaximum particle size of about 2 mm). A sample of FG is analyzed usingX-ray Diffraction (XRD) to identify its crystallographic composition, asshown in FIG. 1. The Rietveld analysis (Young 1993) of the XRD patternindicates that the material quantitatively contains about 74% ofanhydrite (A), about 24% of gypsum (G), about 1% of fluorite (F), andabout 1% of other materials, as reported in Table 1.

TABLE 1 Crystallographic compositions of FG, CFBCA, FA, and PC by weightpercentage (%). FG CFBCA FA PC Components (%) (%) (%) (%) Akermanite:Ca₂Mg(Si₂O₇) — — 32.6 — Alite: 3CaO•SiO₂ — — — 70.4 Anhydrite: CaSO₄74.6 —  6.8 — Brownmillerite: Ca₂(Al,Fe)₂O₅ — — 29.4 23.3 Calcite: CaCO₃— 17.6  — — Ettringite: Ca₆Al₂(SO4)₃(OH)₁₂•26H₂O — 6.8 — — Fluorite:CaF₂  1.0 — — — Gypsum: CaSO₄•2H₂O 24.2 64.9  —  1.4 Periclase: MgO — — 5.9 — Perovskite: CaTiO₃ — —  3.9 — Portlandite: Ca(OH)₂ — 4.1 — —Quartz: SiO₂  0.1 5.9 20.3 — Tricalcium Aluminate: 3CaO•Al₂O₃ — — —  4.9

The CFBCA material was produced by burning petroleum coke, tree bark,and limestone in a boiler used for power generation (Lind 1999). Theprovided material may have a water content of about 20% by weight and apH of about 12.6. The material is air dried and sieved by using the USstandard sieve #10 prior to its use in the experiments. A sample ofCFBCA is analyzed using XRD to identify its crystallographiccomposition, as shown in FIG. 2. The Rietveld analysis of the XRDpattern indicates that the CFBCA contains about 64.9% of gypsum, about17.6% of calcite (C), about 6.8% of ettringite (E), and about 10.7% ofother materials, such as quartz and portlandite.

Table 1 reports the results of a Rietveld analysis of the X-rayDiffraction (XRD) pattern for the FG in slurry form, the U-FG, and theCFBCA. A sample of slurry FG is analyzed using X-ray Diffraction (XRD)to identify its crystallographic composition, as shown in FIG. 1.

The FG used here is slurry FG which is dried and ground before theaddition of CFBCA. The dried slurry FG may have a predominance ofanhydrite (CaSO₄); and the U-FG may have a predominance of gypsum(CaSO₄.2H₂O). This change in crystallographic composition may be due tothe weather exposure of U-FG, rather than to the addition of CFBCA toslurry FG.

Effects of CFBCA on the Properties of C-FG-Based Blends

Two particular compositions of the C-FG-based blends are selected toinvestigate the effects of CFBCA amounts on the mechanical and physicalproperties of interest (Bigdeli et al. 2018b). Hereinafter, eachcomposition is identified by a letter indicating the type of pH-adjustedFG employed (namely, C denotes C-FG and U denotes U-FG), and by threenumbers in parentheses separated by hyphens and indicating the weightpercentages of C-FG/U-FG, FA, and PC, respectively. The two compositionsconsidered are C(60-34-6) and C(62-35-3), and their properties arecompared with those of U(60-34-6) and U(62-35-3).

In order to investigate the effects of the CFBCA content on theproperties of the C-FG-based blends, seven mixtures of FG and CFBCA wereprepared, with amounts of CFBCA (w_(CFBCA)) varying between about 0% andabout 12% with intervals of about 2% (see Table 2). For the U-FG-basedblends, the specific amount of CFBCA could not be determined; however,the amount of CFBCA is between about 2% and about 6% (G. Mitchell, BrownIndustries, personal communication).

TABLE 2 C-FG mixtures used to study the effects of CFBCA content on theproperties of C-FG-based blends. Mixture FG (%) w_(CFBCA) (%) C-FG₁ 1000 C-FG₂ 98 2 C-FG₃ 96 4 C-FG₄ 94 6 C-FG₅ 92 8 C-FG₆ 90 10 C-FG₇ 88 12Specimen Preparation and Testing Procedures

The C-FG mixtures listed in Table 2 are prepared by carefullyproportioning the dried slurry FG and CFBCA passing a US standard sieve#10. Before sieving, the hardened slurry FG is ground and the CFBCA isair dried. The dry C-FG mixtures are then blended with FA and PC toobtain C(62-35-3) and C(60-34-6) compositions. Finally, the material ismixed with water until a uniform paste is obtained, according to theASTM C305-14 standard (ASTM 2014a). Specimen preparation and testingprocedures followed standard methods used for ordinary concrete.

Sets of five cylindrical specimens of about 10.15 cm×about 20.3 cm(about 4 in×about 8 in) size are prepared following the ASTMC192/C192M-16a standard (ASTM 2016a) using different C-FG-based blendsfor all tests of mechanical and durability properties, with theexception of the setting time tests, for which three samples of freshmix are tested per ASTM C403/C403M-08 standard (ASTM 2008). The watercontent of each blend at the curing condition is determined by followingthe ASTM D2216 standard (ASTM 2010). Compressive strength, f_(c), istested following ASTM C39/C39M-16b (ASTM 2016b); chord modulus ofelasticity, E, and Poisson's ratio, v, are tested by following theprocedures described in ASTM C469/C469M-14 (ASTM 2014b); relativevolumetric expansion, η, is measured following the recommendations ofASTM C1005-17 (ASTM 2017); unit weight (density) is obtained accordingto ASTM C642-13 (ASTM 2013); and initial and final setting times aredetermined according to ASTM C403/C403M-08 (ASTM 2008).

Table 3 and Table 4 report the experimental result statistics forcompositions C(62-35-3) and C(60-34-6), respectively, in terms of samplemeans and standard deviations. All results are reported as functions ofthe CFBCA content in the C-FG mix. Table 3 and Table 4 also report thestatistics available for compositions U(62-35-3) and U(60-34-6),respectively. The following subsections discuss the experimentalresults.

TABLE 3 Statistics of mechanical and physical properties for compositionC(62-35-3) made with different C-FG mixes and for compositionU(62-35-3). Compressive Strength Unit Weight w_(CFBCA) μ_(f) _(c) /σ_(f)_(c) μ_(ρ)/σ_(ρ) (%) (MPa) (kg/m³) 0 22.5/0.4 1987/1 2 28.1/0.8 2008/6 430.1/0.8 2049/5 6 32.0/1.1 2041/7 8 33.6/1.0  2048/30 10 33.9/1.5 1983/512 31.5/0.3  1978/27 U-FG  8.9/0.6 1750/7

TABLE 4 Statistics of the mechanical and physical properties forcomposition C(60-34-6) made with different C-FG mixes and forcomposition U(60-34-6). Compressive Strength Unit Weight w_(CFBCA) μ_(f)_(c) /σ_(f) _(c) μ_(ρ)/σ_(ρ) (%) (MPa) (kg/m³) 0 51.5/0.6  2080/15 249.8/0.9 2097/8 4 52.7/2.1 2048/9 6 45.8/1.9  2025/11 8 29.6/1.3 1998/17 10 11.7/0.9 1887/3 12  7.5/0.4 1844/7 U-FG 13.8/1.5 NACompressive Strength

FIG. 3 plots the sample mean of the compressive strength, μ_(f) _(c) ,together with its about 95% CI as a function of w_(CFBCA) forcompositions C(62-35-3) and C(60-34-6). FIG. 3 also reports thecompressive strength sample means, as well as their about 95% CI, forcompositions U(62-35-3) and U(60-34-6), which are reported over therange about 2%˜w_(CFBCA)˜about 6%, because the exact CFBCA content isunknown. It is observed that: (1) the average compressive strength ofcomposition C(62-35-3) slowly increases from about 22.5 MPa to about33.9 MPa for w_(CFBCA) increasing from about 0% to about 10% and thenslightly decreases to about 31.5 MPa for w_(CFBCA)=about 12%; (2) theaverage compressive strength of composition C(60-34-6) is almostconstant for w_(CFBCA)˜4%, reaches a maximum value of about 52.7 MPa atw_(CFBCA)=about 4%, and then decreases dramatically, reaching the valueof about 7.5 MPa for w_(CFBCA)=about 12%; (3) the average compressivestrengths of both compositions U(62-35-3) and U(60-34-6) are equal toabout 8.9 MPa and about 13.8 MPa, respectively, and thus aresignificantly lower (i.e., smaller by a factor greater than 3) thanthose of the corresponding C-FG compositions in the range about2%˜w_(CFBCA)˜about 6%; and (4) the lengths of the about 95% CI for allcompositions are small (i.e., less than about 4 MPa), which indicatesthat the estimates of the average compressive strengths are highlyreliable.

It is concluded that the compressive strength of C-FG-based blends canexperience significant variations for varying amounts of CFBCA anddifferent compositions. It is also concluded that using U-FG has anegative effect on the compressive strength of FG-based blends. Becausethe chemical analysis of the U-FG used for compositions U(62-35-3) andU(60-34-6) did not identify significant amount of impurities, theresults presented here indicate that prolonged weather actions producethis negative effect independently of the amount of CFBCA used toneutralize the FG. It is also observed that, for appropriate values ofw_(CFBCA), the C-FG-based blends considered in this study achievecompressive strengths that are compatible with their use as structuralconstruction materials.

Unit Weight

FIG. 4 plots the sample mean of the unit weight (density), μρ, as afunction of w_(CFBCA) for compositions C(62-35-3) and C(60-34-6), aswell as for composition U(62-35-3). The corresponding 95% CI are alsoreported.

For composition C(62-35-3), μ_(ρ) slightly increases from about 1987kg/m3 to about 2049 kg/m3 for w_(CFBCA) increasing from about 0% toabout 4%, it remains almost constant for about 4%˜w_(CFBCA)˜about 8%(the differences are not statistically significant for α=about 5%), anddecreases to about 1978 kg/m3 for w_(CFBCA)=about 12%. For compositionC(60-34-6), μρ slightly increases from about 2080 kg/m3 to about 2097kg/m3 for w_(CFBCA)=about 0% and about 2%, respectively, although thischange is not statistically significant with a significance level α=5%,and then decreases monotonically to 1844 kg/m3 for w_(CFBCA)=about 12%.The average unit weight of the U-FG-based blend made with compositionU(62-35-3) is about 1750 kg/m3, which is significantly lower than thatof C-FG-based blends.

XRD Results of Raw Materials

A sample of dried slurry FG (FG), i.e., un-treated FG, is analyzed usingX-ray Diffraction (XRD) to identify its crystallographic composition.One representative sample of pH-adjusted FG (U-FG) is also analyzed byXRD method. The results for both materials are reported in Table 5. Acomparison of the results of XRDs for the two materials shows that thedried slurry FG contains about 74.6% anhydrite (CaSO₄) and about 24.2%gypsum (CaSO₄.2H₂O), whereas the pH-adjusted FG (U-FG) contains onlyabout 5.7% anhydrite and about 93.4% gypsum. Without being bound bytheory, this difference may be due to the FG neutralization treatmentand to prolonged exposure to contaminants and environmental phenomena.

TABLE 5 Crystallographic compositions of two types of FG (Slurry FG andpH-adjusted FG, U-FG) and CFBCA (% by dry weight). Slurry pH-adjustedComponents FG FG CFBCA Akermanite: Ca₂Mg(Si₂O₇) — — — Alite: 3CaO•SiO₂ —— — Anhydrite: CaSO₄ 74.6 5.7 — Brownmillerite: Ca₂(Al,Fe)₂O₅ — — —Calcite: CaCO₃ — — 17.6  Ettringite: Ca₆Al₂(SO4)₃(OH)₁₂•26H₂O — — 6.8Fluorite: CaF₂  1.0 0.8 — Gypsum: CaSO₄•2H₂O 24.2 93.4  64.9  Periclase:MgO — — — Perovskite: CaTiO₃ — — — Portlandite: Ca(OH)₂ — — 4.1 Quartz:SiO₂  0.1 0.1 5.9 Calcite: CaCO₃ — — —

Additionally, a sample of CFBCA is analyzed using XRD. The Rietveldanalysis of the XRD pattern indicated that the CFBCA contains about64.9% gypsum, about 17.6% calcite, about 6.8% ettringite, and about10.7% of other materials, such as quartz and portlandite. This materialis used as alkali material for pH-adjustment of the FG.

Compressive Strength Results

The compressive strength of the specimens made of pH-adjusted FG, flyash, and Portland cement (FG, FA, PC), is significantly lower than thatof specimens with the same compositions but made using dried slurry FG.For example, the average compressive strengths of compositions (62-35-3)and (60-34-6) are equal to about 8.9 MPa and about 13.8 MPa,respectively, when made using pH-adjusted FG, and to about 30.1 MPa andabout 52.7 MPa, respectively, when made using dried slurry FG.

The experimental results presented here show that the content of CFBCAused to neutralize the FG material has a significant effect on themechanical and physical properties of C-FG-based blends.

The maximum compressive strength is achieved for an optimal amount ofCFBCA, which depends on the C-FG-based composition and is in the rangeabout 8%˜w_(CFBCA)˜about 10% for composition C(62-35-3) and about2%˜w_(CFBCA)˜about 4% for composition C(60-34-6). These observationssuggest that a good compromise between mechanical properties andproduction cost can be obtained using small amounts of CFBCA (i.e.,w_(CFBCA)˜about 4%). For composition C(60-34-6) it may be advantageousto avoid neutralization of the FG (i.e., to use w_(CFBCA)=about 0%(e.g., 0.01%)), as long as the low pH is not harmful to the equipmentused to grind the dried slurry FG. The results of this investigationalso indicate that composition C(60-34-6) with w_(CFBCA)˜about 4% is apromising material to substitute ordinary concrete in constructionapplications.

Composition U(62-35-3) provides an average compressive strength equal toabout 8.9 MPa, i.e., a reduction in compressive strength of about 68.3%when compared to that of composition C(62-35-3) in the range about 2%w_(CFBCA) to about 6%. Similarly, composition U(60-34-6) has an averagecompressive strength equal to about 13.8 MPa, with a reduction incompressive strength of about 69.9% when compared to that of compositionC(60-34-6) in the range about 2% to about 6% w_(CFBCA). Based also onthe other experimental results, and without being bound by theory, it isconcluded that a prolonged exposure to environmental actions of the U-FGmaterial has a negative effect on the mechanical and durabilityproperties of U-FG-based blends. This effect appears to be significantlylarger than that of different amounts of CFBCA. Therefore, the usage ofC-FG should be preferred to the usage of U-FG in the preparation ofFG-based blends. In addition, the production of C-FG from slurry FG withsmall amounts of CFBCA can potentially be done at lower cost than thecurrent procedure of FG neutralization and stockpiling, due to thefollowing: (1) lower amounts of CFBCA than those needed forneutralization of the slurry FG can be used to produce C-FG-basedblends, (2) the manufacture process can be streamlined by using directlythe dried slurry FG (i.e., avoiding transportation to the stockpile andmultiple grinding phases), and (3) the land use for stockpiling can bereduced.

The experimental results show that the C-FG-based blend made usingcomposition C(60-34-6) with w_(CFBCA) of about 4% is a promisingsustainable substitute of ordinary concrete. This composition achieves acompressive strength between about 49.8 MPa and about 52.7 MPa, which ishigher than the typical range of compressive strength for ordinaryconcrete, i.e., about 20-about 40 MPa (Mehta and Monteiro 2013). Theobserved values of strength indicate that the inventive material areappropriate for structural applications.

The inventive compositions can have a unit weight in the range about2048-about 2080 kg/m³, which is lower than that of normal weightconcrete, i.e., about 2400 kg/m³ (Mehta and Monteiro 2013). The low unitweight of C-FG-based blends is an advantageous property for constructionapplications, because it can reduce the self-weight load. Therefore, theaddition of normal weight and/or normal weight coarse aggregates toC-FG-based blends, could be useful.

Utilization of PC in the inventive compositions, i.e., about 6%, islower than in ordinary concrete, i.e., about 10%-about 15% (Nawy 2000).This property is advantageous, because it indicates that the materialcan be produced at a lower cost and with a lower CO₂ gas release thanordinary concrete, making this material promising as a green substituteof ordinary concrete.

Effects of Composition on the Compressive Strength of C-FG-Based Blends

The effects of composition (in terms of FG, FA, and PC contents) on thecompression strength of C-FG-based blends are investigated using 15different compositions (see Table 6 for percentages of components forthe considered compositions). The experimental investigation exploresthe behavior of C-FG-based blends for about 60%≤FG about ≤90%, FA about≤38%, and about 2%≤PC≤about 10%. Two sets of experiments are conducted(for a total of 30 compositions): (1) 15 compositions using slurry FGwithout neutralization (non-neutralized FG), and (2) the same 15compositions using slurry FG neutralized using lime (neutralized FG).Hereinafter, each composition is identified by a letter C indicatingC-FG, and by three numbers in parentheses separated by hyphens andindicating the weight percentages of C-FG, FA, and PC, respectively.

TABLE 6 Compositions, sample average, and sample standard deviation ofthe compressive strength of C-FG-based blends with non-neutralized FGand FG neutralized with lime. Non- Components amounts for neutralizedNeutralized different compositions FG FG w_(FG) w_(FA) w_(PC) μ_(f) _(c)/σ_(f) _(c) μ_(f) _(c) /σ_(f) _(c) (%) (%) (%) (MPa) (MPa) 60 38 223.1/0.3 23.3/1.0 60 34 6 30.5/1.3 28.6/2.0 60 30 10 14.3/0.3 40.1/1.570 28 2 13.8/0.6 19.1/1.7 70 24 6 24.9/0.5 30.3/2.4 70 20 10 15.9/0.836.2/2.8 80 18 2  9.2/0.1 14.0/1.0 80 14 6 13.2/1.0 23.0/1.6 80 10 1013.7/1.0 26.5/1.8 90 8 2  4.8/0.2  4.3/0.3 90 4 6  6.5/0.1 16.6/1.0 90 010  6.8/0.3  8.9/0.9 73 25 2 13.7/0.7 17.5/1.1 62 35 3 21.6/1.6 22.7/2.375 18 2  9.9/0.5 15.5/0.8Specimen Preparation and Testing Procedures

The 15 C-FG mixtures listed in Table 6 are prepared by carefullyblending the dried slurry FG with FA and PC to obtain differentcompositions of FG-based blends. Before using the dried slurry FG, thehardened slurry FG is ground and is passed through a US standard sieve#10 (about 2 mm). In the case of the neutralized FG, the dried andsieved slurry FG is also mixed with about 0.35% in weight of lime inpowder form before mixing with FA and PC. Finally, the material is mixedwith water until a uniform paste is obtained, according to the ASTMC305-14 standard (ASTM 2014a). Specimen preparation and testingprocedures followed standard methods used for ordinary concrete.

Sets of five equally-built cubic specimens of edge dimension equal toabout 50 mm (about 2 in) are prepared following the ASTM C109/C109M-16astandard (ASTM 2016b) for each considered composition of C-FG-basedblends. The water content of each blend at the curing condition isdetermined by following the ASTM D2216 standard (ASTM 2010). Thecompressive strength, f_(c), is tested following the ASTM C109/C109M-16astandard (ASTM 2016b).

Table 6 reports the experimental result in terms of sample mean, μ_(f)_(c) , and sample standard deviation, σ_(f) _(c) , of the compressivestrength for the different compositions of C-FG-based blends for bothnon-neutralized and neutralized FG cases. The small values of the samplestandard deviations indicate that the tests have a small variation and,thus, a good reproducibility.

Compressive Strength Results for Non-Neutralized FG Case

The results reported in Table 6 for the compositions made usingnon-neutralized FG show that, for FG=about 60% and FG=about 70%, thecompressive strength reaches its maximum value for PC=about 6%, whereasfor FG=about 80% and FG=about 90% the compressive strength increaseswith the amount of PC. It is also observed that for a given percentageof cement content, the average compressive strength of the C-FG-basedblends generally decreases with increasing FG content (with theexception of the average compressive strength for PC=about 10%, which islower for FG=about 60% than for FG=about 70%). The maximum value of thecompressive strength is equal to 30.5 MPa (4,420 psi) and is obtainedfor composition C(60-34-6), whereas the minimum compressive strength isequal to about 4.8 MPa (about 670 psi) and is obtained for compositionC(90-8-2). It is noted here that a compressive strength of about 27.5MPa (about 4,000 psi) or higher is generally considered appropriate forconcrete road pavement construction.

Compressive Strength Results for Neutralized FG Case

The results reported in Table 6 for the compositions made using FGneutralized with lime show that, for FG=about 60%, about 70%, and about80%, the compressive strength increases for increasing PC amounts;whereas for FG=about 90%, the maximum value of the compressive strengthis achieved for PC=about 6%. It is also observed that for a givenpercentage of cement content, the average compressive strength of theC-FG-based blends generally decreases with increasing FG content (withthe exception of the average compressive strength for PC=about 6%, whichis lower for FG=about 60% than for FG=about 70%). The maximum value ofthe compressive strength is equal to about 40.1 MPa (about 5,820 psi)and is obtained for composition C(60-30-10), whereas the minimumcompressive strength is equal to about 4.3 MPa (about 620 psi) and isobtained for composition C(90-8-2).

Example 1

Composition 1 (C(62-35-3)) is made of about 62% C-FG with about 4%circulating fluidized bed combustion ash, about 35% fly ash, and about3% Portland cement. It achieves a characteristic compressive strength ofabout 20 MPa (about 3000 psi) after about 28 days of curing according toASTM C39/C39M-16b and an average elastic modulus of about 13 GPa (about1885 ksi).

Example 2

Composition 2 (C(60-34-6)) is made of about 60% C-FG containing noalkali material, about 34% fly ash, and about 6% Portland cement. Itachieves a characteristic compressive strength of about 40 MPa (about6000 psi) after about 28 days of curing according to ASTM C39/C39M-16band an average elastic modulus of about 21 GPa (about 3045 ksi).

The embodiments illustrated and discussed in this specification areintended only to teach those skilled in the art the best way known tothe inventors to make and use the invention. Nothing in thisspecification should be considered as limiting the scope of the presentinvention. All examples presented are representative and non-limiting.The above-described embodiments of the invention may be modified orvaried, without departing from the invention, as appreciated by thoseskilled in the art in light of the above teachings. It is therefore tobe understood that, within the scope of the claims and theirequivalents, the invention may be practiced otherwise than asspecifically described.

CITATIONS

-   ASTM (2008). ASTM C403/C403M-08 Standard test method for time of    setting of concrete mixtures by penetration resistance. ASTM    International, West Conshohocken, Pa., USA. DOI:    10.1520/C0403-00403M-08.-   ASTM (2010). ASTM D2216-10 Standard test methods for laboratory    determination of water (moisture) content of soil and rock by mass.    ASTM International, West Conshohocken, Pa., USA. DOI:    10.1520/D2216-10.-   ASTM (2013a). ASTM C642-13 Standard test method for density,    absorption, and voids of hardened concrete. ASTM International, West    Conshohocken, Pa., USA. DOI: 10.1520/C0642-13.-   ASTM (2013b). ASTM D4972-13 Standard test method for pH of soils.    ASTM International, West Conshohocken, Pa., USA. DOI:    10.1520/D4972-13.-   ASTM (2014a). ASTM C305-14 Standard test method for mechanical    mixing of hydraulic cement pastes and mortars of plastic    consistency. ASTM International, West Conshohocken, Pa., USA. DOI:    10.1520/C0305-14.-   ASTM (2014b). ASTM C469/C469M-14 Standard test method for static    modulus of elasticity and Poisson's ratio of concrete in    compression. ASTM International, West Conshohocken, Pa., USA. DOI:    10.1520/C0469-00469M-14.-   ASTM (2016a). ASTM C192/C192M-16a Standard practice for making and    curing concrete test specimens in the laboratory. ASTM    International, West Conshohocken, Pa., USA. DOI:    10.1520/C0192-00192M-16A.-   ASTM (2016b). ASTM C39/C39M-16b Standard test method for compressive    strength of cylindrical concrete specimens. ASTM International, West    Conshohocken, Pa., USA. DOI: 10.1520/C0039-00039M-16B.-   ASTM (2017). ASTM C1005-17 Standard test method for reference masses    and devices for determining mass and volume for use in the physical    testing of hydraulic cements. ASTM International, West Conshohocken,    Pa., USA. DOI: 10.1520/C1005-17.-   Azar, D. G. (1990). “Fluorogypsum waste solidification material.”    U.S. Pat. No. 4,935,211. U.S. Patent and Trademark Office,    Washington, D.C., USA.-   Bigdeli, Y., Barbato, M., Gutierrez-Wing, M. T., Lofton, C. D.,    Rusch, K. A., Jung, J., and Jang, J. (2018a). “Development of new    pH-adjusted Fluorogypsum-cement-fly ash blends: Preliminary    investigation of strength and durability properties.” Construction    and Building Materials, 182:644-656.-   Bigdeli, Y., Barbato, M., Gutierrez-Wing, M. T., and Lofton, C. D.    (2018b). “Use of slurry fluorogypsum (FG) with controlled    pH-adjustment in FG-based blends.” Construction and Building    Materials, 163:160-168.    https://doi.org/10.1016/j.conbuildmat.2017.12.099-   Brink, R. H. (1973) “Use of waste sulfate on transpo′72 parking    lot.” Proceedings, Third International Ash Utilization Symposium.    Sponsored by National Coal Association, Edison Electric Institute,    American Public Power Association, National Ash Association, and    Bureau of Mines, Pittsburgh, Pa., USA.-   Chen, S, Rusch, K, Malone, R, Seals, R, Wilson, C, Fleeger, J.    Preliminary evaluation of stabilized phosphogypsum for use within    the aquatic environment. In Proceedings of the Water Environment    Federation, Special Workshop on Food Chain Toxicity-Toxic Substances    in Water Environmental: Assessment and Control. 1995.-   Chesner, W. H., Collins, R. J., and MacKay, M. H. (1998). User    Guidelines for Waste and By-Product Materials in Pavement    Construction, FHWA-RD-97-148, Rept. No. 480017, Turner-Fairbank    Highway Research Center, McLean, Va., USA.-   Ciullo, P. A. (1996). Industrial Minerals and Their Uses: a Handbook    and Formulary. Noyes Publications, Westwood, N.J., USA.-   Clifton, J. R., Brown, P. W., and Frohnsdorff, G. (1980a). “Uses of    waste materials and by-products in construction. Part I.” Resource    Recovery and Conservation, 5(2), 139-160.-   Clifton, J. R., Brown, P. W., and Frohnsdorff, G. (1980b). “Uses of    waste materials and by-products in construction. Part II.” Resource    Recovery and Conservation, 5(3), 217-228.-   CPRA. Integrated ecosystem restoration & hurricane protection in    Coastal Louisiana: Fiscal Year 2014 annual plan. Coastal Protection    and Restoration Authority. 190 p. 2013.-   Deshpande, P S. The determination of appropriate phosphogypsum:    Class C fly ash: Portland type II cement compositions for use in    marine applications. Thesis. Louisiana State University. 2003.-   Eisele, D. J. (2003). “Converting fluorogypsum to calcium sulfate.”    U.S. Pat. No. 6,517,790, U.S. Patent and Trademark Office,    Washington, D.C., USA.-   Fan, Y. Leaching characteristics and structural Integrity of    cement-stabilized phosphogypsum PG. Master of Science thesis. Civil    and Environmental Engineering, Louisiana State University, Baton    Rouge, La. 122.p. 1997.-   Garg, M., and Pundir, A. (2014). “Investigation of properties of    fluorogypsum-slag composite binders-hydration, strength and    microstructure.” Cement and Concrete Composites, 45(2014), 227-233.-   Guo, T. Determination of optimal composition of stabilized PG    composites for saltwater application. Ph.D. thesis. Civil and    Environmental Engineering, Louisiana State University, Baton Rouge,    La. 320.p. 1998.-   Guo, T, Malone, R F, Rusch, K A. Stabilized phosphogypsum: class C    fly ash: Portland type II cement composites for potential marine    application. Environmental Science & Technology, 35(19): 3967-3973.    2001.-   Guo, T, Malone, R F, Seals, R K, Rusch, K A. Determination of    optimal composition of stabilized phosphogypsum composites for    saltwater application. In Hazardous and Industrial    Wastes-Proceedings of the Mid-Atlantic Industrial Waste Conference.    1999.-   Halstead, W. (1979). Potential for Utilizing Industrial Wastes and    By-Products in Construction of Transportation Facilities in    Virginia, FHWA/VA-80/15, National Technical Information Service,    Alexandria, Va., USA.-   King, G. N. (1986). “Method for stabilization of sludge.” U.S. Pat.    No. 4,615,809. U.S. Patent and Trademark Office, Washington, D.C.,    USA.-   Lind, T. (1999). Ash Formation in Circulating Fluidized Bed    Combustion of Coal and Solid Biomass. Ph.D. thesis, Technical    Research Centre of Finland, Espoo, Finland.-   Mehta, P. K., and Monteiro, J. M. (2013). Concrete, Structure,    Properties and Materials. 4th Edition, McGraw-Hill Education, New    York City, N.Y., USA.-   Nawy, E. (2000). Reinforced Concrete: A Fundamental Approach. 4th    edition, Prentice Hall, Upper Saddle River, N.J., USA.-   Nieland, D L, Wilson, C A, Fleecer, J W, Sun, B, Malone, R F,    Chen, S. Preliminary evaluation of the use of phosphogypsum for reef    substrate. I. A laboratory study of bioaccumulation of radium and    six heavy metals in an aquatic food chain. Chemistry and Ecology,    14(3-4): 305-319. 1998.-   Rusch, K. A., Guo, T. Searching for optimum composition of    phosphogypsum:fly ash:cement composites for oyster catch materials:    annual progress report (Project nr 051LSU2759). Gulf Coast Hazardous    Substance Research Center. 2003. 24 pp.-   Rusch, K. A., Seals, R, K., Guo, T. Development of economically    stabilized phosphogypsum composites for saltwater application. FIPR    #01-162-182. Florida Institute of Phosphate Research. 1. 2001. 63    pp.-   Rusch, K A, Seals, R K, Guo, T, Deshpande, P. Development of    economically stabilized phosphogypsum composites for saltwater    application. FIPR #01-162-211. Florida Institute of Phosphate    Research. 2005. 54 pp.-   Rutherford, A. (2011). ANOVA and ANCOVA: a GLM approach. 2nd    edition, John Wiley & Sons, Hoboken, N.J., USA.-   Sajwan, K. S., Alva, A. K., Punshon, T., and Twardowska, I. (2006).    Coal Combustion Byproducts and Environmental Issues, Springer, New    York, N.Y., USA.-   Singh, M., and Garg, M. (2009). “Activation of fluorogypsum for    building materials.” Journal of Scientific and Industrial Research,    68(2), 130.-   UNEP (2005). Solid Waste Management, United Nations Environment    Programme, CalRecovery, Concord, Calif., USA.-   USEPA. National Primary Drinking Water Regulations; Radionuclides;    Final Rule. 40 CFR Parts 9, 141, and 142. F. Register. Vol. 65(236)    76708-76753 p. 2000.-   Usmen, M. A., and Moulton, L. K. (1984). “Construction and    performance of experimental base course test sections built with    waste sulfate, lime, and fly ash.” Transportation Research Record,    998, 52-62.-   Wilson, C A, Fleeger, J W, Malone, R F, Rusch, K A, Seal, R K,    DeLosReyes, J, A. A., Jones, S C, Nieland, D L. The substrate    suitability of phosphogypsum composites for marine habitat    enhancement. Final report. Florida Institute of Phosphate Research.    FIPR 01-127-164. 1998a. 89 pp.-   Wilson, C A, Nieland, D L, Fleecer, J W, Todaro, A, Malone, R F,    Rusch, K A. Preliminary Evaluation of the Use of Phosphogypsum for    Reef Substrate. ii. A Study of the Effects of Phosphogypsum Exposure    On Diversity and Biomass of Aquatic Organisms. Chemistry and    Ecology, 14(3-4): 321340. 1998b.-   Worrell, E., Price, L., Martin, N., Hendriks, C., and Meida, L. O.    (2001). “Carbon dioxide emissions from the global cement industry.”    Annual Review of Energy and the Environment, 26(1), 303-329.-   Wu, Z., Zhang, Z., and Tao, M. (2010). “Stabilizing blended calcium    sulfate materials for roadway base construction.” Construction and    Building Materials, 24(10), 1861-1868.-   Yan, P., and You, Y. (1998). “Studies on the binder of fly    ash-fluorgypsum-cement.” Cement and Concrete Research, 28(1),    135-140.-   Yan, P., Yang, W., Qin, X., and You, Y. (1999). “Microstructure and    properties of the binder of fly ash-fluorogypsum-Portland cement.”    Cement and Concrete Research, 29(3), 349-354.-   Young, R. A. (1993). The Rietveld Method, Oxford: University Press.    ISBN 0-19-855577-6, Oxford, UK.-   Zhang, Z., and Tao, M. (2006). Stability of Calcium Sulfate Base    Course in a Wet Environment, FHWA/LA.06/419, Louisiana    Transportation Research Center, Baton Rouge, La., USA.

The invention claimed is:
 1. A pre-cure composition, comprising: i) adry material comprising fluorogypsum (FG), a pozzolanic material, and ahydraulic cement; and ii) water; wherein the ratio of water to drymaterial ratio is less than about 1/3; and wherein the FG has a pH inthe range of from about 1.5 to about 5.5.
 2. The pre-cure composition ofclaim 1, further comprising an alkali material.
 3. The pre-curecomposition of claim 2, wherein the alkali material is lime.
 4. Thepre-cure composition of claim 1, wherein the pozzolanic material isselected from the group consisting of a circulating fluidized bedcombustion ash (CFBCA), a solid fuel combustion product, a coalcombustion product, fly ash, class C fly ash, class F fly ash, bottomash, flue-gas desulfurization materials, ground blast furnace slag,boiler slag, incinerator bottom ash, a biomass combustion product,bagasse ash, rice hull ash, wood ash, biomass pellets ash, naturalpozzolan, volcanic ash, an industrial amorphous silica product,micro-silica, silica fumes, and combinations thereof.
 5. The pre-curecomposition of claim 1, further comprising an admixture used in theformulation of concrete.
 6. The pre-cure composition of claim 5, whereinthe admixture is selected from the group consisting of a material withlatent hydraulic behavior, a water-reducing agent, entrained air, asuperplasticizer, a set retarding agent, a set accelerating agent, ashrinkage-reducing agent, and combinations.
 7. The pre-cure compositionof claim 1, further comprising an aggregate, wherein the aggregate isselected from a coarse aggregate, a fine aggregate, or a combinationthereof.
 8. The pre-cure composition of claim 1, wherein the hydrauliccement is selected from the group consisting of Portland cement, Type IPortland cement, Type II Portland cement, Type III Portland cement, TypeIV Portland cement, Type V Portland cement, and combinations.
 9. Thepre-cure composition of claim 1, wherein the FG has a pH in the range offrom about 1.5 to about 3.5.
 10. A pre-cure composition comprising:fluorogypsum (FG), hydraulic cement, and a pozzolanic material, whereinsolids in the FG comprises at least about 10 wt % anhydrite (CaSO₄); andwherein the FG has a pH in the range of from about 1.5 to about 5.5. 11.The pre-cure composition of claim 10, wherein solids in the FG compriseat least about 25 wt % anhydrite.
 12. The pre-cure composition of claim11, wherein solids in the FG comprise at least about 50 wt % anhydrite.13. The pre-cure composition of claim 10, wherein the FG has a pH in therange of from about 1.5 to about 3.5.
 14. A pre-cure composition,comprising at least 45 wt % fluorogypsum (FG), at most 10 wt % hydrauliccement, and at most 40 wt % pozzolanic material; wherein the FG has a pHin the range of from about 1.5 to about 5.5; and optionally containingat most 12% of an alkali material.
 15. The pre-cure composition of claim14, further comprising water.
 16. The pre-cure composition of claim 15,wherein the water is present at a water to dry material ratio of lessthan about 1/3.
 17. The pre-cure composition of claim 14, wherein the FGis in slurry form.
 18. The pre-cure composition of claim 17, wherein theslurry FG is wet or dried.
 19. The pre-cure composition of claim 14,wherein the alkali material is lime.
 20. The pre-cure composition ofclaim 14, wherein the FG has a pH in the range of from about 1.5 toabout 3.5.